DAC architecture for LCD source driver

ABSTRACT

A two-stage digital-to-analog converter for outputting an analog voltage in response to a M-bit digital input code includes a two-bit serial charge redistribution digital-to-analog converter having a high reference voltage input node for receiving a high reference voltage and a low reference voltage input node for receiving a low reference voltage, and a voltage selector. The voltage selector sets the high reference voltage and low reference voltage to selected levels depending on at least a portion of the M-bit digital input code.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional of and claims priority to U.S.Provisional Patent Application No. 61/327,147 filed Apr. 23, 2010 andentitled “A New DAC Architecture for LCD Source Driver”, the entirety ofwhich is hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to liquid crystal display (LCD) sourcedrivers, and more particularly to LCD source drivers utilizingdigital-to-analog (DAC) converters.

BACKGROUND OF THE INVENTION

Today's advanced electronics, such as high definition televisions, placeever increasing demands on electronics. For example, customers demandHDTV display systems that can display images with more and more naturalcolors. Typical LCD drivers for driving pixel arrays of an LCD displayuse digital-to-analog converters to convert digital codes representingvoltage levels to corresponding analog outputs. For example, sixteenbinary numbers can be expressed using 4-bits to represent outputvoltages of the DAC. An actual analog output voltage Vout isproportional to an input binary number, and is expressed as a multipleof the binary number. When the reference voltage Vref of the DAC is aconstant, the output voltage Vout has only a discrete value, e.g., oneof 16 possible voltage levels, so that the output of the DAC is nottruly an analog value. However, the number of possible output values canbe increased by increasing the number of bits of input data. A largernumber of possible output values in the output range reduces thedifference between DAC output values.

It should be apparent that when the DAC input includes a relativelylarge number of bits, the DAC provides a relatively high-resolutionoutput. However, the circuit area consumed by the DAC increasesproportionally with resolution. An increase by only 1 bit in resolutiondoubles the area of the decoder in the DAC.

An example of a conventional R-type (resistive string) DAC structureused in a LCD source driver is shown in FIG. 1. More specifically, FIG.1 shows a 6-bit DAC architecture. The DAC structure has a resistivestring coupled between reference voltages V0 to V8. A resistorcombination, and thus the voltage, is selected based on the 6-bitdigital input D0 to D5. An operational amplifier is provided forincreasing the driver current. The 6-bit DAC architecture requires 64resistors, 64 signal lines and one 64x1 decoder. Using this standardarchitecture to fabricate an 8-bit DAC would require a four times (4×)increase in area, i.e., 256 resistors, 256 signal lines and one 256×1decoder. Using this standard architecture, to fabricate a 10-bit DACwould require another four times (4×) increase in area, i.e., 1024resistors, 1024 signal lines and one 1024×1 decoder. Thus, the 10-bitDAC would consume sixteen times as much chip or wafer area than acomparable 6-bit DAC. Traditional DAC architectures take up about 30% ofthe chip or wafer area. At increased resolutions (e.g., 10-bits andbeyond), the size increases needed to achieve these resolutions areunacceptable.

A new DAC architecture for use in high resolution LCD source drivers isdesired.

SUMMARY OF THE INVENTION

A two-stage digital-to-analog converter for outputting an analog voltagein response to a M-bit digital input code includes a two-bit serialcharge redistribution digital-to-analog converter having a highreference voltage input node for receiving a high reference voltage anda low reference voltage input node for receiving a low referencevoltage, and a voltage selector. The voltage selector sets the highreference voltage and low reference voltage to selected levels dependingon at least a portion of the M-bit digital input code.

The above and other features of the present invention will be betterunderstood from the following detailed description of the preferredembodiments of the invention that is provided in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate preferred embodiments of theinvention, as well as other information pertinent to the disclosure.

FIG. 1 is a circuit diagram of a source driver using a prior artresistive string DAC architecture having 6-bit resolution.

FIG. 2 is illustrates a conventional design for a LCD source driver and

FIG. 3 illustrates in more detail the DAC element of the LCD sourcedriver of FIG. 2.

FIG. 4 illustrates a 10-bit DAC architecture in accordance with anembodiment of the present invention and FIG. 4A is a table illustratingthe sequential operation of the DAC architecture of FIG. 4.

FIG. 4B is a table listing the output voltage of the DAC architectureafter each operation illustrated in FIG. 4A.

FIG. 5 illustrates an alternative embodiment of a 10-bit DACarchitecture in accordance with the present invention and FIG. 5A is atable illustrating the sequential operation of the DAC architecture ofFIG. 5.

FIG. 6 illustrates another alternative embodiment of a 10-bit DACarchitecture in accordance with the present invention and FIG. 6A is atable illustrating the sequential operation of the DAC architecture ofFIG. 6.

FIG. 7 illustrates an embodiment of the 10-bit DAC architecture of FIG.4 with built-in offset cancelation and FIG. 7A is a table illustratingthe sequential operation of the DAC architecture of FIG. 7.

FIG. 8 illustrates an embodiment of the 10-bit DAC architecture of FIG.5 with built-in offset cancelation and FIG. 8A is a table illustratingthe sequential operation of the DAC architecture of FIG. 8.

FIG. 9 illustrates an embodiment of the 10-bit DAC architecture of FIG.6 with built-in offset cancelation and FIG. 9A is a table illustratingthe sequential operation of the DAC architecture of FIG. 9.

FIG. 10 illustrates a non-linear example of a transfer curve for asource driver.

FIG. 11 illustrates the gamma correction operation of an embodiment of aDAC in accordance with the present invention.

FIG. 12 illustrates an alternative embodiment of a 10-bit DACarchitecture of FIG. 8 having a modified reference voltage selector.

DETAILED DESCRIPTION

This description of the exemplary embodiments is intended to be read inconnection with the accompanying drawings, which are to be consideredpart of the entire written description. Terms concerning electricalattachments, coupling and the like, such as “connected” and“interconnected,” refer to a relationship wherein structures communicatewith one another either directly or indirectly through interveningstructures, unless expressly described otherwise.

Active-matrix-type liquid crystal displays (hereinafter, will bereferred to as LCDs) are known in the art and described in, for example,U.S. Pat. No. 7,176,869 to Kumada et al., the entirety of which ishereby incorporated by reference herein. The LCD has a gate driver as ascan signal driver for supplying scan signals in a pixel selectionperiod, a source driver as a data signal driver for supplying datasignals to a liquid crystal panel, and a control circuit for controllingtimings for the gate driver and the source driver. These components,except for the improvements to the source driver described herein, areknown in the art and need not be described in detail herein.

In the liquid crystal display, graphic data is transmitted from thecontrol circuit to the source driver where the graphic data signal isconverted from digital to analog and supplied to the liquid crystalpanel as its drive voltage. A reference voltage generator circuitconnected to the source driver produces a voltage that serves as areference in the D-to-A conversion of the graphic data signal.

FIG. 2 is a schematic illustration of a conventional LCD source driver10. The source driver 10 includes a digital part implemented in a lowvoltage (LV) technology. This part includes a shift register 12, asampling register 14, a hold register 16 and a data latch 18. The analogpart, which is implemented in a higher voltage or voltages, includes alevel shifter 20, a DAC 22, a reference voltage generator 24 and anoutput circuit 26, which may include operational amplifiers as shown inFIG. 3. The output of the driver 10 is shown as having 720 analogoutputs Y1 to Y720, one each for each line of an LCD display.

FIG. 3 is a more detailed schematic illustration of the DAC 22 and oneform of output circuit 26 of the of the source line driver 10 of FIG. 2.The DAC and output circuit architecture are typically constructed as adifferential architecture, including alternating NMOS and PMOS based DACstructures 22 a, 22 b, respectively, and PMOS and NMOS input operationalamplifiers 26 a, 26 b, respectively. However, rather than a differentialarchitecture, those familiar with such designs will understand that arail-to-rail operational amplifier output circuit architecture may beused. There may be several drivers in a LCD display. For example, forHDTV 1920×1080, there may be 8 drivers in the display (1920×3(RGB)/720).The operations of the LCD source driver and its components illustratedin FIGS. 2 and 3 are familiar to those of ordinary skill in this art. Assuch, a detailed description of these components is not needed and isnot provided so as to avoid obscuring the description of the presentinvention, which relates to improved DAC architectures for use in suchLCD drivers.

The improved DAC architecture illustrated herein breaks the DACfunctionality into two stages. A first stage provides a coarse outputvoltage range corresponding roughly to an M-bit digital input code, anda second stage uses a two-bit serial charge redistribution DAC toprovide the finer target voltage within the coarse range. Gammacorrection and offset cancellation can be built into the DACarchitecture. As will be apparent from the following description, theDAC architecture can provide significant area savings for high speed,large panel, high resolution designs.

FIG. 4 illustrates a first embodiment of a high resolution DACarchitecture 100, and FIG. 4A illustrates the operational stepsperformed by the DAC in producing an analog voltage Vout from a M-bitdigital input code. More specifically, FIG. 4 illustrates an embodimentof a 10-bit DAC architecture 100. While a 10-bit embodiment isillustrated, it should be understood that the general principlesillustrated by the 10-bit embodiment apply equally to DAC architecturesof higher resolutions (e.g., 11-bit and higher designs) and even tothose of lower resolution (9-bit and lower designs) if desired toimplement as such.

The 10-bit DAC architecture 100 includes an output operational amplifier102, which is provided for current gain purposes. The output of theoperational amplifier 102 (Vout) is fed back to the negative input ofthe operational amplifier 102. The positive input of the operationalamplifier 102 is coupled to the output of a serial charge-redistributionDAC 104, specifically a 2-bit serial charge redistribution DAC, which isdiscussed in more detail below. The serial charge-redistribution DAC 104has high reference voltage and low reference voltage inputs forreceiving a pair of reference voltages VH and VL, which define a coursevoltage range. Voltage selector circuit 106 provides reference voltagesVH and VL, which in the illustrated embodiment are an adjacent voltagepair selected by the voltage selector 106 from a plurality of adjacentvoltage pairs spanning references voltages V1 to V9. A 10-bit input coderanging from least significant bit (LSB) d0 to most significant bit(MSB) d9 is provided to Code Expanding & Decision logic 112. Assumingvoltage selector 106 selects amongst Y adjacent pairs of voltages, theCode Expanding & Decision logic 112 extracts the log₂Y-most significantbits from the 10-bit input code. For example, if there are eight voltagepairs from V1 to V9 in the embodiment of FIG. 4 (i.e. V1/V2, V2/V3,V3/V4, V4/V5, V5/V6, V6/V7, V7/V8 and V8/V9), then the Code Expanding &Decision logic 112 extracts the three most significant bits (d9, d8, d7)from the 10-bit input code for use in selecting an adjacent pair ofvoltages. The Code Expanding & Decision logic 112 provides those threebits to temporary storage, such as to a register 110. The three mostsignificant bits are provided to a first decoder 108 for decoding into acontrol signal for controlling the voltage selector 106 to output one ofeight possible VL and VH pairs corresponding to the three bit input codeto the decoder 108. For example, if the [d9 d8 d7] is [1 1 1], then theVL/VH pair is V8/V9, and if [d9 d8 d7] is [0 0 0], then the VL/VH pairis V1/V2. With the coarse voltage range represented by VL and VH, thetwo-bit serial charge-redistribution DAC 104 is used to output thespecific voltage level within the range of VL to VH corresponding to the10-bit input code, as discussed below.

FIG. 4 illustrates an embodiment described as an N=1 embodiment. Thatis, the Code Expanding & Decision logic expands the 10-bit digital inputcode by 1 bit. In the illustrated embodiment, the expansion bit is usedas a filler or padding bit after the least significant bit of the 10-bitdigital input code. This bit is set to a default of “0”. The seven leastsignificant bits (d6 to d0) and the one filler or padding bit, for atotal of eight bits, are provided from Code Expanding & Decision logic112 to second register 116. An N=0 embodiment is contemplated when thenumber of least significant bits provided by Code Expanding & Decisionlogic 112 is an even number, e.g., 8-bits in an 11-bit digital inputcode embodiment. Where the filler bit is always set to 0, no gammacorrection (discussed below) is implemented via the Code Expanding &Decision logic provided by the 10-bit architecture. In an N=1 embodimentwith gamma correction, the expansion bit could be dynamically set toeither “0” or “1” by the logic 112.

This 8-bit code (d6 d5 d4 d3 d2 d1 d0 0) is provided to temporarystorage register 116. Register 116 is controlled tosequentially/serially provides the stored 8-bit code to the seconddecoder 114 in a sequence of two bit combinations [dH dL], starting fromthe least significant bits of the eight bit code, i.e., combination [d00] first, then combination [d2 d1] second, then combination [d4 d3]third and finally combination [d6 d5] last. These code combinations areused by the second decoder 114 to control the two-bit serial chargeredistribution DAC 104.

Two-bit serial charge-redistribution DAC 104 operates to select avoltage within the range of VL to VH for output to the operationalamplifier 102. The charge-redistribution DAC 104 includes a terminationcapacitor C3 connected between the low reference voltage node and acharge collection node 109 coupled to the positive input of theoperational amplifier 102 and a pair of binary weighted capacitors C1,C2, each having a first end also coupled to the low reference voltagenode and second ends coupled to first capacitor charging node 105 andsecond capacitor charging node 107, respectively. The second end ofcapacitor C1 is selectively coupled to either the low reference voltageVL or the high reference voltage VH during a charging cycle via a firstswitching circuit, which includes a switch S1 and a pair ofcomplementary switches SH, SH bar. A second end of capacitor C2 isselectively coupled to either the low reference voltage VL or the highreference voltage VH during a charging cycle via a second switchingcircuit, which also includes a switch S1 and a pair of complementaryswitches SL, SL bar. Complementary switches SH, SH bar and complementaryswitches SL, SL bar are controlled by the output of the second decoder114.

The first capacitor charging node 105 is coupled to the chargecollection node 109 via a switch S2 during a charge redistributioncycle, and the second capacitor charging node 107 is coupled to thecharge collection node 109 via a second switch S2 during the chargeredistribution cycle. A switch S3 is coupled between the low referencevoltage node and the charge collection node 109 for purpose of resettingthe capacitor voltages during a reset operation. Switches S1, S2 and S3can be controlled in any number of ways, such as by clock signals issuedby a clock controller.

For an individual two bit combination [dH dL], when dH is a “1” then theswitch SH is closed and the switch SH bar is open, and when dH is a “0”then the switch SH is open and the switch SH bar is closed. Similarly,when dL is a “1” then the switch SL is closed and the switch SL bar isopen, and when dL is a “0” then the switch SL is open and the switch SLbar is closed.

Capacitors C2 and C3 have capacitance value C and capacitor C2 hascapacitor value 2C. As should be apparent, the charge within a capacitoris a multiple of the capacitance of the capacitor. So, assuming forexample that both C1 and C2 are charged at the same time, the chargewithin capacitor C1 will be twice that of the charge within C2.

The operation of the serial charge-redistribution DAC 104 is illustratedwith the aid of FIG. 4A.

At Step 1, switches S1 are open and switches S2 and S3 are both closed.This step resets the voltage across capacitors C1, C2 and C3 to 0V, asboth electrodes of each capacitor are coupled to voltage VL. After step1, switch S3 is opened and left open until the routine is run again anda reset of capacitor C3 is required.

At Step 2, switches S1 are closed and switches S2 are open for purposesof charging capacitors C1 and C2. The first 2-bit combination [dH dL],i.e., [d0 0] provided by the register 116, is used by the second decoder114 to control switches SH, SH bar, SL and SL bar. If dH is “1”, then SHis closed, SH bar is open and capacitor C1 is coupled between VH and VLfor charging. If dH is “0”, then SH is open, SH bar is closed andcapacitor C1 is coupled VL and VL and not charged. If dL is “1”, then SLis closed, SL bar is open and capacitor C2 is coupled between VH and VLfor charging. If dL is “0”, then SL is open, SL bar is closed andcapacitor C2 is coupled between VL and VL, meaning it is not charged.

At Step 3, switches S1 are open and switches S2 are closed for purposesof distributing any charge built-up in capacitors C1 and C2, and anyresidual charge in capacitor C3 (which is none at this point), betweencapacitors C1, C2 and termination/collection capacitor C3. Specifically,closing switches S2 connects capacitors C1, C2 and C3 together inparallel between charge collection node 109 and the low referencevoltage node. The total charge in the circuit is distributed such thatthe charge in each capacitor is proportional to its capacitance. Thatis, capacitor C1 has half the total charge (Q_(total)) and each ofcapacitors C1 and C3 have one-quarter of the total charge, since totalcapacitance is 4C. The charge distributed to capacitor C3 results in avoltage at the output node equal to VL+V_(C3). The voltage V_(C3) isequal to (Q_(total))/4C. During each charge distribution phase/cycle,there is a distribution to capacitor C3 of ¼ of the total charge in thecircuit. After this step, the output node voltage is equal to(2d+0)/4*(VH−VL)+VL. At this step, the register 116 also loads the next2-bit combination [d2 d1] to the second decoder 114 in preparation forthe next capacitor charge phase/cycle.

Step 4 operates in the same manner as step 2 only with switches SH, SHbar, SL and SL bar under control of the second instance of thesequential 2-bit code, i.e., combination [d2 d1]. Depending on thevalues of [d2 d1], step 4 can add charge to the charge already existingin capacitors C1 and C2. At step 5, capacitors C1, C1 and C3 are againconnected in parallel between the low reference voltage node and node109. The total charge Q_(total) in the circuit includes the residualcharges in capacitor C3 (at the end of step 3) plus the total charge incapacitors C1 and C2 (i.e., the residual charge at the end of step 3 inthese capacitors plus any charge added to those capacitors in step 4).The total charge Q_(total) is again redistributed across the threecapacitors in proportionate shares. This results in a residue voltagedivision by a factor of 4 in capacitor C3. The voltage at the outputnode is again equal to VL+V_(C3). After step 5, VL+V_(C3) is equal tothe total charge in capacitor C3 divided by the total combinedcapacitance 4C of the capacitors C1/C2/C3. After step 5, the output nodevoltage is equal to: (2d2+d1+0.5d0)/4*(VH−VL)+VL.

The operation of Steps 6-9 should be apparent from the foregoingdescription of steps 2-5. The resulting voltage at the node 109 for eachstage is shown in FIG. 4B. As shown in FIG. 4B, the voltage at theoutput node at this time is1/128(64d6+32d5+16d4+8d3+4d2+2d1+d0)*(VH−VL)+VL. That is, the voltagecould be anywhere from VL (if all eight bits of the code provided toregister 116 are “0”) to VL+127/128 (VH−VL) (if the seven mostsignificant bits of the code provided to register 116 are “1” and thefiller bit is 0).

The output voltage provided by the two-bit serial charge redistributionDAC architecture conforms to the following summation formula, where “n”and “i” represent differential variables and where when i=1 then direpresents d1, if i=2, then di represents d2, etc.

$V_{L} + {\left( {V_{H} - V_{L}} \right)*{\sum\limits_{i = 0}^{n}{\left( {2^{i}{di}} \right)*{2^{- {({n + 1})}}.}}}}$

While the serial charge redistribution DAC architecture is describedherein as being a 2-bit serial charge redistribution DAC, it should beunderstood that the architecture can be upwardly scaled as needed toaccommodate higher order resolutions. For example, a 3-bit serial chargeredistribution DAC could have an additional binary weighted capacitor ofcapacitance 4C coupled in the same manner as capacitors C1 and C2 andcontrolled by a separate switching circuit. The decoder 114 would beconfigured as a three-bit decoder and register 116 would provide threebit combinations rather than two-bit combinations.

FIGS. 5 and 5A illustrate an alternative DAC architecture 100A and thesequential operation thereof, respectively. The architecture 100A isidentical in all respects to architecture 100 except that N=2. That is,the remaining seven least significant bits of the 10-bit input code areexpanded to nine bits by the addition of two (N=2) extra bits d00 andd01. Code Expanding and Decision logic 112A determines the value ofthese two bits, as described in more detail below, and provides theseven bits d6 to d0 from the original input code with these twoadditional bits d00 and d01 to the register 116A. Register 116A thenprovides to second decoder 114A 2-bit combinations derived from the9-bit code provided by the Code Expanding & Decision logic 112Asequentially and in two-bit combinations [dH dL], least significant bitsfirst, in the manner described above in connection with FIG. 4 forcontrol of switches SH, SH bar, SL and SL bar. That is, register 116Afirst provides [d00 0], with the “0” in the dL location being a fillerbit; then [d0 d01]; then [d2 d1]; then [d4 d3]; and finally [d6 d5].

The charge redistribution circuit 104 of FIG. 5 is structurally the sameas the corresponding circuit from FIG. 4. The only operationaldifference is the addition of extra charge and redistribution Steps 10and 11 as shown in FIG. 5A and the respective bit combinations used atthe individual steps, i.e., FIG. 5A begins with the [dH dL] bitcombination [d00 0] rather than [d0 0] as in FIG. 4A.

FIGS. 6 and 6A illustrate an alternative DAC architecture 100B and thesequential operation thereof, respectively. The architecture 100B isidentical in all respects to the architectures 100 and 100A except thatN=3. That is, the remaining seven least significant bits of the 10-bitinput code are expanded to ten bits by the addition of three (N=3) extrabits d00, d01 and d02. Code Expanding and Decision logic 112B determinesthe value of these three bits, as described in more detail below, andprovides the seven bits d6 to d0 from the original 10-bit input codewith these three additional bits d00, d01 and d02 to the register 116B.Register 116B then provides to second decoder 114B 2-bit combinationsderived from the 10-bit code provided by the Code Expanding & Decisionlogic 112B sequentially and in two-bit combinations [dH dL], leastsignificant bits first, in the manner described above in connection withFIGS. 4 and 5 for control of switches SH, SH bar, SL and SL bar. Thatis, register 116B first provides [d01 d00]; then [d0 d02]; then [d2 d1];then [d4 d3]; and finally [d6 d5]. Note that the need for a filler bit“0” in this embodiment.

The charge redistribution circuit 104 of FIG. 6 is structurally the sameas the corresponding circuits from FIGS. 4 and 5. The only operationaldifference from the steps illustrated in FIG. 5A is found in steps 1 to4, which use the first two 2-bit codes of FIG. 6 rather than those ofFIG. 5.

FIG. 7-9 illustrate embodiments of the DAC architectures illustrated inFIGS. 4-6 but modified to provide offset compensation. The modifiedoperation of these architectures are illustrated in connection withFIGS. 7A, 8A and 9A, respectively. Unless described otherwise, thestructure and operations of these DAC architectures are identical tothose of FIGS. 4 to 6 and 4A to 6A, respectively.

FIG. 7 shows a DAC architecture 200 and FIG. 7A shows sequential stepsillustrating the operation of the DAC architecture of FIG. 7. The DACarchitecture 200 is identical to the DAC architecture 100 of FIG. 4except for charge redistribution circuit 104A. Compared to chargeredistribution circuit 104, charge redistribution circuit 104A includes:an additional switch S2 coupled between node 109 and the positive inputof the operational amplifier 102; an additional switch S2 coupledbetween the output of the operational amplifier 102 and a node 111; aswitch S4 coupled between nodes 109 and 111; and a fourth capacitor C4coupled between node 111 and the positive input of the operationalamplifier 102. These additional components operate to compensate for anyoffset voltage that may be inherent in the operational amplifier 102.

Referring now to FIG. 7A, steps 1-9 of FIG. 7A are identical to thosedescribed above in connection with FIG. 4A. That is, steps 1-9 areperformed to charge capacitor C3 to a desired voltage, which is added tothe lower voltage VL. This voltage (VL+V_(C3)) is the voltage at node109 in FIG. 7 after step 9 is performed. It should be noted that forsteps 1-9 switch S4 is open, meaning capacitor C4 is not connected tonode 109. Added third and fourth switches S2 are open during steps 2, 4,6 and 8. This disconnects capacitor C4 from the circuit when capacitorsC1 and C2 are charging. However, during the redistribution steps 3, 5, 7and 9, these additional switches S2 are triggered, which couples node109 to the positive input of operational amplifier and creates afeedback path from the output of the operational amplifier 102 into thepositive input of the operational amplifier 102 through capacitor C4.This connection stores the offset voltage (Vos) of the operationalamplifier 102 into the capacitor C4. At this step, the output voltagefrom the operational amplifier 102 is equal to the voltage at node 109minus the offset voltage (Vos) of the operational amplifier 102. FIG. 7Aillustrates an additional step S10 is performed after capacitor C3 isfully charged (Step 9). Step 10 is an offset cancelation step. At step10, only switch S4 is triggered, which connects node 109 to the positiveinput of the operational amplifier 102 through node 111 and capacitorC4. As noted, the voltage across capacitor C4 represents the offsetvoltage (Vos) of the operational amplifier 102. This offset voltage isadded to the voltage at node 109 to compensate for the offset providedby operational amplifier 102. As such, the output voltage Vout from theoperational amplifier 102 more closely matches the voltage at node 109.That is, the output voltage Vout from the operational amplifier 102equals: the voltage at node 109 (V₁₀₉)+Vos−Vos, i.e., V₁₀₉.

FIG. 8 shows an alternative DAC architecture 200A and FIG. 8A showssequential steps illustrating the operation of the DAC architecture 200Aof FIG. 8. The DAC architecture 200A is identical to the DACarchitecture 100A of FIG. 5 except for use of modified two-bit serialcharge redistribution DAC 104A described above in connection with FIG.7A. As noted above, the modified charge redistribution DAC 104A hasbuilt-in offset cancellation. Referring now to FIG. 8A, steps 1-11 ofFIG. 8A are identical to those described above in connection with FIG.5A. That is, steps 1-11 are performed to charge capacitor C3 to adesired voltage, which is added to the lower voltage VL. This voltage(VL+V_(C3)) is the voltage at node 109 in FIG. 8 after Step 11 isperformed. The operation of Step 12 in performing offset cancelation isthe same as the operation of step 10 of FIG. 7A described above.

FIG. 9 shows another alternative DAC architecture 200B and FIG. 9A showssequential steps illustrating the operation of the DAC architecture 200Bof FIG. 9. The DAC architecture 200B is identical to the DACarchitecture 100B of FIG. 6 except for use of modified two-bit serialcharge redistribution DAC 104A described above in connection with FIG.7A. Referring to FIG. 9A, steps 1-11 of FIG. 9A are identical to thosedescribed above in connection with FIG. 6A. That is, steps 1-11 areperformed to charge capacitor C3 to a desired voltage, which is added tothe lower voltage VL. This voltage (VL+V_(C3)) is the voltage at node111 in FIG. 9 after Step 11 is performed. The operation of Step 12 inperforming offset cancelation is the same as the operation of step 10 ofFIG. 7A described above.

FIG. 12 illustrates an alternative DAC architecture 200C, which likearchitecture 200A of FIG. 8A is a 10-bit DAC with built-in offsetcanceling and N=2. The DAC architecture 200A is identical to the DACarchitecture 200A of FIG. 8A except that decoder 108 and register 110are eliminated; voltage selector 106 is replaced with modified voltageselector 106A; Code Expanding & Decision logic 112A is replaced withCode Expanding & Decision logic 112C; register 116A is replaced withregister 116C; and second decoder 114A is replaced with second decoder114C.

In this embodiment, VL and VH are each adjustable to one of twodifferent levels via voltage selector 106A. Voltage selector 106Areceives as inputs high supply voltage VDD, common mode voltage VCOM,and low power supply voltage VSS, as polarity control signal POL. Thevoltage selector can be viewed functionally as a one bit decoder forselecting between adjacent voltage pairs VSS/VCOM and VCOM/VDD. Inputsignal POL to the voltage selector is a polarity signal and can be usedto select voltage pair VDD/VCOM (corresponding to a positive polaritysignal POL (i.e., POL=1)) or voltage pair VCOM/VSS (corresponding to anegative polarity signal POL (i.e., POL=0)). Signal POL can be generatedin any number of ways known to those familiar with differential logiccircuits such as timing control circuits.

As with FIG. 8, the 10-bit input code is provided to Code Expanding &Decision logic 112C. The logic 112C expands the 10-bit code by two bitsto twelve bits and provides this 12-bit code to register 116C. Register116C provides the code serially in two increments [dH dL] to the seconddecoder 114C for control of the two-bit serial charge redistribution DAC104A. The operation of the serial charge redistribution DAC 104A isidentical to that described above in connection with FIG. 8 only nofiller bit is required and with use of an additional charge andredistribution steps associated since there are six [dH dL] bitcombinations rather than five.

Further details of the Code Expanding & Decision logic described aboveare discussed below in connection with FIGS. 10 and 11. As is recognizedin the art, a LCD converts a video signal to light in a nonlinear way,because the transfer curve of the liquid crystal display, voltage vs.light transmitting, is nonlinear. The gamma characteristic is apower-law relationship that approximates the relationship between theencoded luma (black/gray/white information) in a video signal and theactual desired image luminance. The LCD display will typically applysome gamma characteristic to the video signal. As such, gamma inversionis applied to the output voltage levels to neutralize the gammacharacteristic and provide or approach a linear relationship betweenencoded luma and the actual image luminance. FIG. 10 illustrates anexample of a transfer curve for a source driver. The Y-axis representsthe voltage and the X-axis represents the input code. The region fromGMA0˜GMA1 is positive polarity and GMA2˜GMA3 is negative polarity. Thecurve illustrates that there are regions in the gamma curve where therelationship is linear and regions where the relationship is nonlinear.The Code Expanding & Decision logic described herein expands theoriginal input 10-bit code by N-number of bits (e.g., from 10 bits to 12bits as shown in the illustration of FIG. 10). The preferred N value is1, 2 or 3, though the invention is not so limited. The extra N-bit(s)are used to provide for adjustment to the code to account for whetherthe transition from a given voltage level to the next voltage level islinear or nonlinear. As shown in FIG. 10, by adding two bits theoriginal 10-bit input code for 1 (0000000001) becomes the 12-bit codefor 4 (000000000100); the original 10-bit input code for 2 (0000000010)becomes the 12-bit code for 8 (0000000001000); the original 10-bit inputcode for 3 (0000000011) becomes the 12-bit code for 12 (0000000001100);etc. In the linear region, a straight code conversion is appropriate,i.e., the 10-bit code for 512 (1000000000) becomes the 12-bit code for2048 (100000000000). However, in the non-linear region, some adjustmentto the code is required to account for the non-linearity. For example,the expanded 12-bit code corresponding to the original 10-bit input codefor 1 (0000000001) becomes the 12-bit code for 4 but adjusted by somevalue +/−K. That is, depending on the value of K, the adjusted expandedcode could be: (000000000001) (i.e., K=−3) (000000000010) (i.e., K=−2)(0000000000110) (i.e., K=−1); (000000000100) (i.e., K=0); (000000000101)(i.e., K=1); (000000000110) (i.e., K=2); or (000000000111) (i.e., K=3).

The transfer curve shown in FIG. 10 is nonlinear between code 0 andcodes 1/2/3, and linear between code 512 and 513. It should beunderstood that the transfer curve of FIG. 10 is only illustrative ofone example of a transfer curve and that individual LCD displays may beassociated with respective individual transfer curves.

The Code Expanding & Decision logic described above is responsible for(1) expanding the input code by N-number of bits (i.e., from 10 to 12bits), and (2) determining the proper adjustment (by a value K) to theresulting code as appropriate to achieve desired transfer curve. Thisprocess as part of a digital-to-analog conversion is illustrated by FIG.11.

At step 300, the M-bit (e.g., 10-bit) input code is received by the CodeExpanding & Decision logic.

At step 310, the Code Expanding & Decision logic expands the code from Mbits to M+N total bits.

At step 320, the proper output code is derived for a specified gammacurve. If the code is in the linear region of the LCDvoltage-transmittance curve, then [code_(i+1)−code_(i)] (M+Nbits)=[code_(j+1)−code_(j)]×2^(N)(M bits), where “j” represents a codenumber in the original code and “i” represents the corresponding codenumber in the expanded code. In the linear region, the code numberdifference between the adjacent expanded codes is simply weighted by2^(N) to the original code. For example, if the curve is linear betweenthe second and third codes for an N=2 embodiment, the second code is 4and the third code is 8. However, if the code is in the nonlinearregions of the LCD V-T curve, then [code_(i+1)−code_(i)] (M+Nbits)=[code_(j+1)−code_(j)]×2^(N)±k (M bits). The code number differencebetween the adjacent expanded codes will have the 2^(N) weighting to theoriginal code but also an adjustment (+/−k) for nonlinear fitting. Theadjustment depends on the V-T curve of the LCD and the Code Expanding &Decision logic may utilize a look up table or registers to store theselected proper code and/or the proper offset. It should be understoodthat “k” is not the same for each expanded code and its value depends onthe nonlinear curve.

It should be understood that as part of the code expansion/decisionprocess the non-linear gamma curve can be fitted roughly by theadjustment of selectable voltage pairs V1˜V9.

At illustrated by step 330, the outputted M+N-bit code is used by the2-bit serial DAC portion of the DAC architecture, along with theselected voltage pair (VH/VL), as described above in connection withFIGS. 4-9, to provide the gamma corrected output voltage Vout.

The DAC architecture disclosed herein can significantly reduce the DACarea for high resolution DAC architectures, such as those used in LCDsource drivers. For example, it is believed that for a 10-bit DACarchitecture, the DAC architecture disclosed herein reduces the DAC areaby at least 50% over a 10-bit DAC implemented using a conventional DACarchitecture. The DAC architecture is well suited to high speed, largepanel, high resolution displays.

Although the invention has been described in terms of exemplaryembodiments, it is not limited thereto. Rather, the appended claimsshould be construed broadly to include other variants and embodiments ofthe invention that may be made by those skilled in the art withoutdeparting from the scope and range of equivalents of the invention.

What is claimed is:
 1. A two-stage digital-to-analog converter foroutputting an analog voltage in response to a M-bit digital input code,comprising: a two-bit serial charge redistribution digital-to-analogconverter having a high reference voltage input node for receiving ahigh reference voltage and a low reference voltage input node forreceiving a low reference voltage; and a voltage selector, the voltageselector setting the high reference voltage and low reference voltage toselected levels depending on at least a portion of the M-bit digitalinput code; wherein the two-bit serial charge redistributiondigital-to-analog converter includes: an output operational amplifierhaving a first operational amplifier input coupled to an output of theoutput operational amplifier and a second operational amplifier inputcoupled to a charge collection node; a termination capacitor coupledbetween the charge collection node and the low reference voltage inputnode; a first capacitor coupled between the low reference voltage inputnode and a first capacitor charging node; a second capacitor coupledbetween the low reference voltage input node and a second capacitorcharging node; a first switching circuit for coupling the firstcapacitor charging node to one of the low reference voltage input nodeand the high reference voltage input node during capacitor charge cyclesin response to instances of a two-bit control code from a sequence oftwo-bit control codes derived from the M-bit digital input code; asecond switching circuit for coupling the second capacitor charging nodeto one of the low reference voltage input node and the high referencevoltage input node during the capacitor charge cycles in response to theinstances of the two-bit control code; and a third switching circuit forcoupling the first and second capacitor charging nodes to the chargecollection node during charge redistribution cycles following thecapacitor charge cycles, wherein the first and second capacitors arebinary weighted with respect to one another.
 2. The two-stagedigital-to-analog converter of claim 1, wherein the voltage selector isconfigured to select for the high and low reference voltages a pair ofadjacent reference voltages from a plurality of pairs of adjacentreference voltages.
 3. The two-stage digital-to-analog converter ofclaim 2, wherein the plurality of pairs of adjacent reference voltagesincludes 8 pairs of adjacent reference voltages, and wherein the voltageselector selects the pair of adjacent reference voltages dependent uponthe three most significant bits of the M-bit digital input code.
 4. Thetwo-stage digital-to-analog converter of claim 1, wherein the voltageselector is configured to set the low and high reference voltages to alow power supply voltage and to a common mode voltage, respectively, orto the common mode voltage and a high power supply voltage,respectively.
 5. The two-stage digital-to-analog converter of claim 1,further comprising built-in offset cancelation for said outputoperational amplifier.
 6. The two-stage digital-to-analog converter ofclaim 1, further comprising gamma correction expansion and decisionlogic responsive to the M-bit digital input code for implementing gammacorrection through code expansion.
 7. The two-stage digital-to-analogconverter of claim 6, wherein the gamma correction expansion anddecision logic expands the M-bit digital input code by one, two or threebits.
 8. The two-stage digital-to-analog converter of claim 1, whereinthe voltage selector is configured to select for the high and lowreference voltages a pair of adjacent reference voltages from aplurality of pairs of adjacent reference voltages, wherein the pluralityof pairs of adjacent reference voltages includes Y pairs of adjacentreference voltages, and wherein the voltage selector selects the pair ofadjacent reference voltages dependent upon the X-most significant bitsof the M-bit digital input code, where X is equal to log₂Y, wherein thetwo-stage digital-to-analog converter further comprises: logic forselecting the X-most significant bits of the M-bit digital input code;an X-bit decoder, the decoder decoding the selected X-most significantbits for controlling the voltage selector; and logic for selecting theZ-least significant bits of the M-bit digital input code, where Z isequal to M-X; a register for temporarily storing at least the Z-leastsignificant bits and providing two-bit combinations of bits derived fromat least the Z-least significant bits; and a two-bit decoder having aninput coupled to the register and an output coupled to the two-bitserial charge redistribution digital-to-analog converter.
 9. Thetwo-stage digital-to-analog converter of claim 8, wherein M is equal to10 and X is equal to
 3. 10. The two-stage digital-to-analog converter ofclaim 8, further comprising code expansion and decision logic, includingsaid logic for selecting the Z-least significant bits, responsive to theM-bit digital input code for implementing gamma correction through codeexpansion.
 11. The two-stage digital-to-analog converter of claim 10,wherein the gamma correction expansion and decision logic expands theM-bit digital input code by one, two or three bits.
 12. The two-stagedigital-to-analog converter of claim 1, further comprising a resetswitch coupled between the low reference voltage input node and thecharge collection node.
 13. The two-stage digital-to-analog converter ofclaim 1, wherein the two-bit serial charge redistributiondigital-to-analog converter further includes means for offsetcancelation.
 14. The two-stage digital-to-analog converter of claim 13,wherein the offset cancelation means includes: an offset cancelationcapacitor having a first end coupled to the second operational amplifierinput and a second end coupled to an intermediate node; a fourthswitching circuit configured to connect the charge collection node tothe second operational amplifier input, and the intermediate node to theoutput of the output operational amplifier, during charge redistributioncycles; and a switch configured to connect the intermediate node to thesecond end of the offset cancelation capacitor during an offsetcancelation operation after a final charge redistribution cycle.
 15. Aliquid crystal display (LCD) source driver including: a two-stagedigital-to-analog converter for outputting an analog voltage in responseto a M-bit digital input code, the two-stage digital-to-analog convertercomprising: a two-bit serial charge redistribution digital-to-analogconverter having a high reference voltage input node for receiving ahigh reference voltage and a low reference voltage input node forreceiving a low reference voltage; a voltage selector, the voltageselector setting the high reference voltage and low reference voltage toselected levels depending on the M-bit digital input code; and gammacorrection expansion and decision logic responsive to the M-bit digitalinput code for implementing gamma correction through code expansion,wherein the two-bit serial charge redistribution digital-to-analogconverter includes: an output operational amplifier having a firstoperational amplifier input coupled to an output of the outputoperational amplifier and a second operational amplifier input coupledto a charge collection node; a termination capacitor coupled between thecharge collection node and the low reference voltage input node; a firstcapacitor coupled between the low reference voltage input node and afirst capacitor charging node; a second capacitor coupled between thelow reference voltage input node and a second capacitor charging node; afirst switching circuit for coupling the first capacitor charging nodeto one of the low reference voltage input node and the high referencevoltage input node during capacitor charge cycles in response toinstances of a two-bit control code from a sequence of two-bit controlcodes derived from the M-bit digital input code; a second switchingcircuit for coupling the second capacitor charging node to one of thelow reference voltage input node and the high reference voltage inputnode during the capacitor charge cycles in response to the instances ofthe two-bit control code; and a third switching circuit for coupling thefirst and second capacitor charging nodes to the charge collection nodeduring charge redistribution cycles following the capacitor chargecycles, wherein the first and second capacitors are binary weighted withrespect to one another.
 16. The LCD source driver of claim 15, furthercomprising built-in offset cancelation for said output operationalamplifier.
 17. The LCD source driver of claim 15, wherein the gammacorrection expansion and decision logic expands the M-bit digital inputcode by one, two or three bits.
 18. The LCD source driver of claim 17,wherein M is equal to 10 or more.
 19. A liquid crystal display (LCD)source driver including: a two-stage digital-to-analog converter foroutputting an analog voltage in response to a M-bit digital input code,the two-stage digital-to-analog converter comprising: a two-bit serialcharge redistribution digital-to-analog converter having a highreference voltage input node for receiving a high reference voltage anda low reference voltage input node for receiving a low referencevoltage; a voltage selector, the voltage selector being configured toselect for the high and low reference voltages a pair of adjacentreference voltages from a plurality of pairs of adjacent referencevoltages, the plurality of pairs of adjacent reference voltagesincluding Y pairs of adjacent reference voltages, and the voltageselector selecting the pair of adjacent reference voltages responsive tothe X-most significant bits of the M-bit digital input code, where X isequal to log₂Y; gamma correction expansion and decision logic responsiveto the M-bit digital input code for implementing gamma correctionthrough code expansion, the logic providing an expanded code from theZ-least significant bits of the M-bit digital input code, where Z isequal to M−X; and a two-bit decoder providing control signals to thetwo-bit serial charge redistribution digital-to-analog converter inresponse to a sequential series of two-bit combinations from theexpanded code.
 20. A two-stage digital-to-analog converter foroutputting an analog voltage in response to a M-bit digital input code,comprising: a two-bit serial charge redistribution digital-to-analogconverter having a high reference voltage input node for receiving ahigh reference voltage and a low reference voltage input node forreceiving a low reference voltage; and a voltage selector, the voltageselector setting the high reference voltage and low reference voltage toselected levels depending on at least a portion of the M-bit digitalinput code; wherein the voltage selector is configured to select for thehigh and low reference voltages a pair of adjacent reference voltagesfrom a plurality of pairs of adjacent reference voltages, wherein theplurality of pairs of adjacent reference voltages includes Y pairs ofadjacent reference voltages, and wherein the voltage selector selectsthe pair of adjacent reference voltages dependent upon the X-mostsignificant bits of the M-bit digital input code, where X is equal tolog₂Y, wherein the two-stage digital-to-analog converter furthercomprises: logic for selecting the X-most significant bits of the M-bitdigital input code; an X-bit decoder, the decoder decoding the selectedX-most significant bits for controlling the voltage selector; and logicfor selecting the Z-least significant bits of the M-bit digital inputcode, where Z is equal to M−X; a register for temporarily storing atleast the Z-least significant bits and providing two-bit combinations ofbits derived from at least the Z-least significant bits; and a two-bitdecoder having an input coupled to the register and an output coupled tothe two-bit serial charge redistribution digital-to-analog converter.